As Fairfield County kindergartens open for another new school year, the area's suburban districts will see older student bodies than urban schools.

An average of one in three kindergartners in Bridgeport Public Schools were 4 years old at the beginning of September, according to an analysis of student data for the previous five school years; in Darien, one in nine kindergartners were 4 years old.

While the differences are significant, they are not surprising. Keeping younger children out of kindergarten for a year to give them more time to mature is a normal practice for parents in affluent Fairfield County towns and cities.

The practice is called "redshirting," a term taken from college athletics, which refers to students who sit out their first year in an attempt to extend their period of eligibility.

In academic redshirting, parents choose to wait an extra year to put their child into kindergarten to give them time to mature physically, emotionally and intellectually, in an effort to help them rise to the top of their class in their primary and secondary careers.

According to a pair of researchers at the National Bureau of Economic Research, 96 percent of 6-year-olds were enrolled in first grade in 1968, but that percentage dropped over the next four decades.

In 2005, 84 percent of 6-year-olds were in first grade, as an increasing number of children began kindergarten later, according to researchers David Deming and Susan Dynarski.

For Helene Neigler, whose twin daughters, Brooke and Mia, will turn 5 in November, the decision to enroll the girls in pre-kindergarten this year rather than sending them to kindergarten at Stamford's Westover Magnet Elementary School was an easy one.

"In my thought, it's going to prepare them for school, because it gives them an extra year of confidence and social skills," she said, noting she started her 8-year-old son, Aiden, a year after he became eligible for kindergarten.

She said it was more about who the girls will be as eighth-graders, high school seniors and young women than who they are now as 4-year-olds.

"We're thinking about their lifetime. And when they go to college, they could go anywhere in the country," she said.

Connecticut has the latest birthdate cutoffs for kindergarteners in the nation. While state law states students are to begin kindergarten the year they turn 5, even if their birthday isn't until Dec. 31, 27 states have birthday cutoffs in September, and several others have even earlier deadlines, according to the Education Commission of the States.

So if Brooke and Mia, who were born in November, were to enter kindergarten this year and rolled through the grades on schedule, they would be 17 their first year of college and wouldn't turn 21 until halfway through their senior year. Many of their classmates from other states would be close to a year or more older than they are.

Connecticut's State Board of Education proposed moving the kindergarten cutoff date forward. That would mean students would need to turn 5 by Oct. 1 to enter kindergarten starting in 2014.

"They tried to change the entry age and it got shot down, mostly because it impacts the poorer kids who don't have the opportunity to attend a preschool program for another year," said Michelle Sabia, a curriculum associate for early childhood education at Stamford Public Schools.

Neigler describes her decision to enroll her daughters for a pre-kindergarten program at the Jewish Community Center's Sara Walker Nursery School in Stamford as "a gift." Like most gifts, it comes with a price tag: A year's tuition for each child at the preschool is around $8,000, too high for many Fairfield County families.

"Children who have the opportunity to attend private preschool programs are going to be the ones who have the opportunity of redshirting," said Sabia. "For our poorer children, who are going to (public) early childhood programs or have no preschool experience, their families can't afford to send them for another year."

In Fairfield County, Bridgeport had the highest percentage of 4-year-old kindergartners of the eight districts analyzed, which include Bridgeport, Darien, New Canaan, Wilton, Fairfield, Greenwich, Stamford and Norwalk. It also has the highest percentage of students living in poverty. But while Bridgeport and Darien are on two opposite ends of the socio-economic spectrum, the contrast in their kindergarten students' ages wasn't unique to the comparison between the two districts. Norwalk had the second-highest percentage of 4-year-olds in kindergarten at 30.7 percent, followed by Stamford at 29.4 percent, putting three of the county's districts at one end of the line. Darien had the smallest percentage of 4-year-olds, followed by the wealthier towns of New Canaan at 13.3 percent and Wilton at 16.7 percent.

The difference is even more striking when comparing fall babies across Fairfield County. In Darien, 38.7 percent of October babies enrolled in kindergarten when they are 4 years old, rather than holding off for a year through redshirting between 2007 and 2011; in Stamford, 88 percent of October babies started kindergarten at 4 years old.

"Redshirting is a big trend here in Darien. We're lucky that financially here, it's not oftentimes a consideration. But I know in other towns it is," said Sharon Patricelli, co-president of the parent-teacher organization at Darien's Ox Ridge Elementary School who has three children with birthdays in the last third of the year.

"I think it seems like . . . about half the kids had done the extra year and half hadn't. it seemed like there was kind of a half and half split. I'm not sure that they really had a huge advantage. But for our own reasons, it was the right thing to do," Patricelli said.

Patricelli's oldest child, Anna, will be 12 in early September when she begins seventh grade. When the Patricelli's decided to enroll her, they decided against redshirting her. But for her two younger children, Reed, a fourth-grader who will be 10 on Monday, and Alex, a second-grader who will be 8 in December, redshirting was the way to go, she said.

"I think for our youngest son, who is very small, he's one of the smallest kids in his whole grade -- boys and girls -- and he'll fight that challenge all the way through. If he was going into third grade now at the size he is going into second grade, he'd be disadvantaged," she said.

As it was, Alex was one of 322 5-year-olds entering kindergarten in Darien at the beginning of the 2010-11 school year. There were only 23 4-year-olds, as well as 20 6-year-olds and one 7-year-old. That same year in Stamford, 362 4-year-olds, 917 5-year-olds and 30 6-year-olds enrolled in kindergarten.